Europe's Galileo satellite navigation system took another step forward on Sunday with the launch of a second experimental satellite.

The Giove-B carries an atomic clock that is stable to within 1 nanosecond per day making it the most accurate atomic clock ever flown into space, according to the European Space Agency. It was launched from the Baikonur cosmodrome in Kazakhstan at 4:16am local time on Sunday (10:16pm GMT Saturday).

Precise timekeeping is vital to the accuracy of satellite positioning because ground receivers work out their position by measuring and comparing the time it takes for signals to be received from several satellites.

An error of 1 nanosecond in timing means a positioning error of 30 centimeters. Clocks need to be kept perfectly in-sync if the system is to pinpoint a user's position with useful accuracy.

The previous testbed satellite, Giove-A, had a pair of atomic clocks with 10-nanosecond accuracy.

When the Galileo navigation system begins operating officially each of the 30 satellites will carry two of the 1-nanosecond accurate clocks for primary timekeeping and two of the less accurate 10-nanosecond clocks as backups.

The Galileo project was launched by the European Union to provide a civil alternative to the US military-controlled Navstar GPS system. Although the US system is available for commercial use, European nations wanted their own system that could be relied upon even in times of war or dispute. While Galileo and Russia's GLONASS are intended to provide an alternative to GPS the three systems are interoperable.

The project is also getting on firmer financial footing. Last week the European Parliament agreed to provide 3.4 billion euros (US$5.3 billion) to see the system launched by 2013. Galileo had been in doubt after a consortium of private companies backed out of the project last November but the public money should ensure the system is launched.

The next satellites scheduled to be launched, in 2010, are the first four of the Galileo constellation.

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